Yes it is not wrong in law. Not quiet sure what is the recommended action to take once you stop play clearly "for injury" and after seeing the injury 20 seconds later, you think/know that it was at least a foul. If you didn't see it then I'd say stay with the original decision. If you did see it but you were day dreaming at that moment and now the replay in your head tells you it was a foul, what do you do? How can you sell it?
The only way I can think of selling it is going over to the assistant and having a chat, but then you are killing him as everyone will think he alerted you to it. Especially in this situation when it is on the far side to the active assistant.